27. LATE MIOCENEHOLOCENE PALEOCEANOGRAPHY OF THE
WESTERN EQUATORIAL ATLANTIC: EVIDENCE FROM DEEP-SEA BENTHIC
FORAMINIFERS1

Hisato Yasuda2

ABSTRACT

Changes in deep-water environments of the western Atlantic
over the past six million years were investigated by studying
benthic foraminiferal abundance changes in 335 samples from Sites
926 (3598 m), 928 (4010 m), and 929 (4358 m) on the Ceara Rise.
The location of these sites on a transect at lower abyssal depths
provides a unique opportunity to evaluate changes in water mass
properties as well as in the supply of organic matter from the
surface waters, especially because other Leg 154 studies provided
a precise time scale for correlations between the sites.

Relative abundances and fluxes of the most abundant species
were calculated, with emphasis on the four most common
speciesNuttallides umbonifera, Globocassidulina
subglobosa, Epistominella exigua, and Alabaminella
weddellensis. There are two patterns in relative abundance
of these species with time: the first group of species (N.
umbonifera and G. subglobosa) shows abundance
variations that differ from site to site (i.e., with depth). The
other group (E. exigua and A. weddellensis)
shows variations in relative abundance that are similar at all
three sites and thus independent of depth.

In the first group, G. subglobosa increased in
relative abundance at the shallowest Site 926 from 2.4 to 1 Ma,
but decreased at the two deeper sites. In contrast, N.
umbonifera, a marker species for corrosive bottom waters and
specifically for Antarctic Bottom Water (AABW), shows large
fluctuations in relative abundance, but overall increased in
relative abundance at the two deeper sites and decreased at the
shallower site from 2.1 Ma to the present. Its relative abundance
shows three peaks during the last 0.9 m.y., which were coeval at
the two deeper sites. These large fluctuations in abundance of N.
umbonifera suggest that production of AABW fluctuated in
intensity, with increases during interglacial periods. AABW
formation increased overall from about 3 Ma, and the amplitude of
the fluctuations increased from 0.9 Ma to present. The
differences between sites are interpreted as showing that the
deeper Sites 928 and 929 were commonly within AABW, whereas the
shallower site was not.

The species in the second group, E. exigua and A.
weddellensis, occur in the recent oceans commonly in oceanic
regions where spring blooms lead to seasonal deposition of
phytodetrital material. At the Ceara Rise sites, they show
variations in relative abundance that are similar at all three
sites from 4.5 to 1.2 Ma, confirming that these species react to
environmental factors that are not depth related, and thus
possibly to surface productivity. The two species, however, do
not covary exactly: at 2.6 Ma, for example, E. exigua decreased
in relative abundance whereas A. weddellensis increased.